Automobile tires, which are formed from rubber compositions containing natural rubber and synthetic diene rubbers as raw materials, can be degraded in the presence of ozone and eventually cracked. In order to reduce formation and growth of cracks in the presence of ozone, the rubber compositions contain additives, including, for example, antioxidants such as N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) or poly(2,2,4-trimethyl-1,2-) dihydroquinoline (TMDQ), and petroleum-derived waxes.
The antioxidants and petroleum-derived waxes in rubber vulcanizates migrate (bloom) to the rubber surface of e.g. tires, thereby protecting the rubber from ozone. However, excessive blooming of the antioxidants and petroleum-derived waxes in short periods of time causes white discoloration. Moreover, the antioxidants degraded by ozone cause brown discoloration and similarly can excessively bloom, resulting in a more intense brown discoloration. In addition, if the waxes and other agents bloomed to the surface of a tire form surface irregularities, diffused reflection of light will occur, with the result that the brown discoloration caused by the degraded antioxidants may be more noticeable, and moreover the tire will lose its shine.
Patent Literature 1 discloses the incorporation of a polyoxyethylene ether nonionic surfactant to prevent deterioration of tire appearance. Unfortunately, this surfactant can accelerate cure to cause problems such as scorch. Thus, there is still room for improvement in terms of the balance between processability and discoloration resistance.